From the viewpoint of the improvements in automobile fuel efficiency and crash safety of the automobiles, there have recently been increasing demands for car body materials to be increased in strength for thickness reduction in order to reduce the weight and increase the strength of car bodies themselves. For this purpose, the use of high strength steel sheets in automobiles has been promoted.
In general, automotive steel sheets are painted before use. As a pretreatment before painting, a chemical conversion treatment called phosphatization is performed. The chemical conversion treatment for steel sheets is one of the important treatments for ensuring corrosion resistance after painting.
The addition of silicon is effective for increasing the strength and the ductility of steel sheets. During continuous annealing, however, silicon is oxidized even if the annealing is performed in a reductive N2+H2 gas atmosphere which does not induce the oxidation of Fe (which reduces Fe oxides). As a result, a silicon oxide (SiO2) is formed on the outermost surface of a steel sheet. This SiO2 inhibits a reaction for forming a chemical conversion film during a chemical conversion treatment, thereby resulting in formation of a microscopical region where any chemical conversion film is not generated. (Hereinafter, such a region will be sometimes referred to as “non-covered region”.) That is, chemical convertibility is lowered.
Among conventional techniques directed to the improvement of chemical convertibility of high-Si containing steel sheets, patent document 1 discloses a method in which an iron coating layer is electroplated at 20 to 1500 mg/m2 onto a steel sheet. However, this method entails the provision of a separate electroplating facility and increases costs correspondingly to an increase in the number of steps.
Further, patent documents 2 and 3 provide an improvement in phosphatability by specifying the Mn/Si ratio and by adding nickel, respectively. However, the effects are dependent on the Si content in a steel sheet, and a further improvement will be necessary for steel sheets having a high Si content.
Patent document 4 discloses a method in which the dew-point temperature during annealing is controlled to be −25 to 0° C. so as to form an internal oxide layer which includes a Si-containing oxide within a depth of 1 μm from the surface of a steel sheet base as well as to control the proportion of the Si-containing oxide to be not more than 80% over a length of 10 μm of the surface of the steel sheet. However, the method described in patent document 4 is predicated on the idea that the dew-point temperature is controlled with respect to the entire area inside a furnace. Thus, difficulties are encountered in controlling the dew-point temperature and ensuring stable operation. If annealing is performed while the controlling of the dew-point temperature is unstable, the distribution of internal oxides formed in a steel sheet becomes nonuniform to cause a risk that chemical convertibility may be variable in a longitudinal direction or a width direction of the steel sheet (non-covered regions may be formed in the entirety or a portion of the steel sheet). Even though an improvement in chemical convertibility is attained, a problem still remains in that corrosion resistance after electrodeposition coating is poor because of the presence of the Si-containing oxide immediately under the chemical conversion coating.
Further, patent document 5 describes a method in which the steel sheet temperature is brought to 350 to 650° C. in an oxidative atmosphere so as to form an oxide film on the surface of the steel sheet, and thereafter the steel sheet is heated to a recrystallization temperature in a reductive atmosphere and subsequently cooled. With this method, however, it is often the case that the thickness of the oxide film formed on the surface of the steel sheet is variable depending on the oxidation method and that the oxidation does not take place sufficiently or the oxide film becomes excessively thick with the result that the oxide film leaves residue or is exfoliated during the subsequent annealing in a reductive atmosphere, thus resulting in a deterioration in surface quality. In EXAMPLES, this patent document describes an embodiment in which oxidation is carried out in air. However, oxidation in air causes a problem such as giving a thick oxide which is hardly reduced in subsequent reduction or requiring a reductive atmosphere with a high hydrogen concentration.
Furthermore, patent document 6 describes a method in which a cold rolled steel sheet containing, in terms of mass %, Si at not less than 0.1% and/or Mn at not less than 1.0% is heated at a steel sheet temperature of not less than 400° C. in an iron-oxidizing atmosphere to form an oxide film on the surface of the steel sheet, and thereafter the oxide film on the surface of the steel sheet is reduced in an iron-reducing atmosphere. In detail, iron on the surface of the steel sheet is oxidized at not less than 400° C. using a direct flame burner with an air ratio of not less than 0.93 and not more than 1.10, and thereafter the steel sheet is annealed in a N2+H2 gas atmosphere which reduces the iron oxide, thereby forming an iron oxide layer on the outermost surface while suppressing the oxidation of SiO2 which lowers chemical convertibility from occurring on the outermost surface. Patent document 6 does not specifically describe the heating temperature with the direct flame burner. However, in the case where Si is present at a high content (generally, 0.6% or more), the oxidation amount of silicon, which is more easily oxidized than iron, becomes large so as to suppress the oxidation of Fe or limit the oxidation of Fe itself to a too low level. As a result, the formation of a superficial reduced Fe layer by the reduction becomes insufficient and SiO2 comes to be present on the surface of the steel sheet after the reduction, thus possibly resulting in a region which may not be covered with a chemical conversion film.